Coke is an indispensable ingredient of many industrial and metallurgical operations, for example, blast-furnace, certain smelting operations and certain chemical and purification processes. In order to be able to control the end result of such industrial operations which use coke, it is very essential that the coke should be of uniform predictable quality and meet certain characteristic requirements.
Additionally, the need for a smokeless fuel which would not obnoxiously pollute controlled environments, especially in the face of oil shortages or high price of crude oil, has largely been responsible for the emphasis on the development of sophisticated coke-ovens which produce high quality coke of predictable coke-characteristics such as porosity, volatile content, coherence, swelling, coke-reactivity, mechanical strength and combustibility. The aforesaid coke-characteristics in part are influenced by the coal-grade per se, and partly the nature and extent of initial preparation which the raw coal is put through prior to coking; largely however, coke-characteristics for a given grade of coal and initial preparation are influenced by the control of operating conditions relating to a coke oven where coke is made from coal and influenced as well as coke quenching.
Significantly, control of the wet-quenching operation of coke is quite a crucial step in coke production for the reasons explained later in this text.
Coke ovens are of many types; in the basic or fundamental form, a coke oven is known to be built in the form of a firebrick chamber in a substantially hemispherical shape. Coke ovens of such type are termed "beehive coke ovens". Invariably nowadays with very minor exceptions, beehive coke ovens being old fashioned and somewhat wasteful are obsolete presently; instead, "byproduct ovens" are used for many applications.
Coal is charged into the top of empty coke ovens mechanically and levelled to a uniform layer by a mechanical rake or other means, as the first step in coke-production. Almost immediately after charging because of the heat retained in the oven from previous charges, or because of preheating, gases start evolving from the charged coal. The evolved gases start burning because of combustion air admitted in controlled quantities. More recently, effluent gases because of the aforesaid combustion are collected whereby a portion of the heat contained therein is utilized for generating steam, through the use of a waste-heat boiler.
The mechanical rake or the other means referred to above is usually in the form of a levelling bar which is mounted a pusher machine. The pusher machine invariably is a structure steel carriage which carries a pushing `ram`, the levelling bar and usually a pusher door-extractor; the pusher machine advantageously runs on rails which run along a battery of coke-ovens, and is located wherever desired, opposite to the oven to be pushed. The `ram` generally consists of a long heavy steel girder (for example, over 50 feet long) terminating in a heavy cast iron head. Preferably, the ram is also provided with a rack and pinion so that the ram may be moved using a motor. When pushing an oven, the head of a ram is moved gently against the coke and the charge is forced out of the oven by means of the ram through a coke guide into a wet-quenching car, which is sometimes referred to as a transfer car.
During wet-quenching of hot coke, coke absorbs water owing to the increased penetration of the quenching water into the pores in the hot coke, unlike in the case of coke at ordinary temperatures. When the quenching water enters the coke pores and the coke temperature decreases as a result of quenching, stresses occur which release particles from the coke mass; these particles end up either as dust or pulverized coke in the quenching carriage. In either event, the particles result in coke in certain amounts lost in the process. It is therefore desirable to perform quenching as conservatively as possible, notwithstanding the fact that requirements of metallurgical plants for coke of particular sizes have become less stringent because coke with a grain size of over 25 mm is considered suitable for iron production, for example, in many types of blast furnace.
On the other hand, the requirements for a specific final water content of the coke are just as stringent as ever. Buyers demand a prescribed maximum final water content in the coke partly because, exceeding the specified maximum final water content results in loss of useful coke mass and specific heating energy. The water content would also influence essential desirable features of porosity, mechanical strength, combustibility, coherence, etc. to various degrees. The attainment of a relatively low final water content in the coke, while desirable, is however more difficult with fine coke than with coarse coke in the quenching process.
To keep the final water content as low as possible and also as uniform as possible in the various grain-size distributions of the coke, a prior art process is known, which is referred to as dump-quenching. In such process, hot coke is quenched on the quenching carriage or a transfer car at time intervals with quenching water during predetermined individual quenching phases. However, such a prior art process has the disadvantage that high emissions occur, and a portion of the coke is lost in the form of dust-emission.
Recently, however, stringent regulations have been applied to industry in general and in particular to coking plants in terms of emissions and effluents such as resulting from quenching, and the regulations may not be violated in order to satisfy the requirements of environmental protection agencies. These regulations generally cannot be met using the known quenching towers of the prior art which are used in large numbers in coking plants because, the quenching vapors entrain too much coke dust. Therefore, to comply with emission regulations, the coke-dust particles must first be totally separated from the effluents by installations in the tower chimney adjacent to the quenching hood. However, the conformity with the emission requirements can be accomplished only to an unsatisfactory degree, if at all, with existing quenching towers despite separation of coke dust by commercially available means. The presently imposed stringent environmental regulations generally require erecting new quenching towers wherein the vapor velocity can be reduced by increasing the crucial tower cross-sections; alternatively, sophisticated installations can be provided with which fewer dust particles are generated. However, it is extremely expensive to build new quenching towers to replace the existing ones from the point of view of high installation costs and additionally the down time needed to tear down the old ones and erect new quenching towers.
The purpose of this invention is to provide wet-quenching of coke in such a way that low emission limits are maintained, using a very economical simple and reliable arrangement. By far, the most desirable arrangement for wet quenching would obviously produce the least amount of wasteful coke dust and would simultaneously provide very uniform wet-quenching from one batch to the next batch of coke produced whereby the percentage of final water content is controlled to be within limits and is substantially uniform for all batches of coke produced within a single production schedule. Such a desirable arrangement would have to be able to control the causes which produce excessive coke dust, excessive final water content and nonuniformity of final water content from batch to batch.
It was found that the efficacy of a well-controlled wet-quenching process depended on the water pressure head throughout each quenching operation, and a critical water-quantity vs. time relationship or profile for each quenching cycle.
In addition to considerations of maintaining a predetermined water pressure head and a known water quantity vs. time relationship, it has been experimentally determined by applicant that wet-quenching of each batch in two discrete consecutive phases would produce the minimum of wasteful coke dust and the least amount of absorbed water.
According to this invention, the first phase comprises supplying quenching water at a relatively higher rate for a short period of time, followed by a relatively low amount of quenching water supplied for a longer time period for the second and final phase.
Since the amount of quenching water is very quickly raised to its maximum value after the quenching carriage is set off in the initial phase of coke quenching, the dust particles are washed out of the generated vapor clouds in a very dense cloud, which prevents dust particles from entering the region of the quenching tower above the quenching hood. In the substantially longer final phase following the initial phase, the amount of water is greatly reduced; dust particles are no longer formed after the hot-coke surface is initially quenched. In the final phase, since the cooled coke is not wetted as much, the customer-requirements for a low final water content can easily be maintained and even surpassed.
The present invention accordingly provides a novel wet-quenching method for coke wherein quenching is performed in at least two distinct stages or phases. In the initial phase, a high rate of water flow under a constant pressure head from an overhead tank is used for a short period of time; in the second and final phase, a low rate of water flow is maintained for a longer period of time under a constant pressure of water; owing to the lower rate of water flow in the second stage, it is relatively less difficult to have a fine control over the maximum final water content.
As to the question of using quenching water supply under a constant head, it is conceivable that pressure regulators and other even more sophisticated devices for controlling pressure head and the total water quantity can be used; however, such devices are amenable to variations and disturbances in the system and are additionally uneconomical. The present invention expediently uses overhead tanks which are filled and kept ready for discharging during quenching, for the quenching operation of each load of coke; thus, the pressure head for the water jets in the first and second stages is determined entirely by the level of water in the overhead tanks and is absolutely independent of supply pressure variations without having to use sophisticated and expensive pressure regulators. Furthermore, the amount of water that can empty to quench each load of coke is limited by the water in storage in the tank. Two significant variables namely, the water pressure head and the quantity of water are controlled in a simple and reliable manner in the invention. The transition from the first quenching phase to the second quenching phase from the point of view of controlling or changing the water flow rate is advantageously achieved by using fast acting valves in the system.